With 4 months advanced tape time they've had plenty of time to do the story before Casey even thought about leaving. They didn't bother.

Even with 4 months of advance film time they're not going to waste their time paring a couple when half of that couple won't be around in the near future. It would just be a bunch of build up with no payoff when Chad leaves. What they should have done is not waste Chad on Melanie. But Melanie was the female version of her father and everyone man just had to love her. They had their chance to fully explore Chabby and they wasted it (and Chad) on Melanie. With 4 months left to film I imagine their focus was on writing a proper exit for the character. And even if they did recast the character, there's no guarantee that the new guy will work out. Remember when they recast Philip? 'Nuff said.